EAGAIN、EWOULDBLOCK、EINTR与非阻塞 长连接
EWOULDBLOCK用于非阻塞模式，不需要重新读或者写
EINTR指操作被中断唤醒，需要重新读/写在Linux环境下开发经常会碰到很多错误(设置errno)，其中EAGAIN是其中比较常见的一个错误(比如用在非阻塞操作中)。
从字面上来看，是提示再试一次。这个错误经常出现在当应用程序进行一些非阻塞(non-blocking)操作(对文件或socket)的时候。例如，以 O_NONBLOCK的标志打开文件/socket/FIFO，如果你连续做read操作而没有数据可读。此时程序不会阻塞起来等待数据准备就绪返 回，read函数会返回一个错误EAGAIN，提示你的应用程序现在没有数据可读请稍后再试。
又例如，当一个系统调用(比如fork)因为没有足够的资源(比如虚拟内存)而执行失败，返回EAGAIN提示其再调用一次(也许下次就能成功)。
Linux - 非阻塞socket编程处理EAGAIN错误
在linux进行非阻塞的socket接收数据时经常出现Resource temporarily unavailable，errno代码为11(EAGAIN)，这是什么意思？
这表明你在非阻塞模式下调用了阻塞操作，在该操作没有完成就返回这个错误，这个错误不会破坏socket的同步，不用管它，下次循环接着recv就可以。 对非阻塞socket而言，EAGAIN不是一种错误。在VxWorks和Windows上，EAGAIN的名字叫做EWOULDBLOCK。
另外，如果出现EINTR即errno为4，错误描述Interrupted system call，操作也应该继续。
最后，如果recv的返回值为0，那表明连接已经断开，我们的接收操作也应该结束。
转载自：http://blog.chinaunix.net/uid-23849526-id-120552.html
static void lx_recv_callback(struct ev_loop *loop, ev_io *w, int revents)
{char buffer[4096] = { 0 };int ret = 0;//ev_io write_event;192.168.206.129
loop:ret = recv(w->fd, buffer, LX_MAXLEN, 0);if (ret > 0) {//lx_print("thread[%lu] recv message :%s  \n", pthread_self(), buffer);gstTcpDevice.parse.unpackFunc(&(gstTcpDevice.parse), (uint8_t *)buffer, ret);} else if (ret == 0) {gstTcpDevice.is_need_reconnect = 1;//lx_error("thread[%lu] remote socket closed!socket fd: %d, errno=%d\n", pthread_self(), w->fd, errno);return;} else {if (errno == EAGAIN || errno == EWOULDBLOCK) {goto loop;}if (errno == ETIMEDOUT) { //client close//lx_error("thread[%lu] ret :%d ,close socket fd : %d, errno:%d, client cannot connect\n", pthread_self(), ret, w->fd, errno);gstTcpDevice.is_need_reconnect = 1;return;} else {//lx_error("thread[%lu] ret :%d ,close socket fd : %d, errno:%d\n", pthread_self(), ret, w->fd, errno);gstTcpDevice.is_need_reconnect = 1;return;}}
}

errno对照表

errno0 :     Successerrno1 :     Operation not permittederrno2 :     No such file or directoryerrno3 :     No such processerrno4 :     Interrupted system callerrno5 :     Input/output errorerrno6 :     No such device or addresserrno7 :     Argument list too longerrno8 :     Exec format errorerrno9 :     Bad file descriptorerrno10 :    No child processeserrno11 :    Resource temporarily unavailableerrno12 :    Cannot allocate memoryerrno13 :    Permission deniederrno14 :    Bad addresserrno15 :    Block device requirederrno16 :    Device or resource busyerrno17 :    File existserrno18 :    Invalid cross-device linkerrno19 :    No such deviceerrno20 :    Not a directoryerrno21 :    Is a directoryerrno22 :    Invalid argumenterrno23 :    Too many open files in systemerrno24 :    Too many open fileserrno25 :    Inappropriate ioctl for deviceerrno26 :    Text file busyerrno27 :    File too largeerrno28 :    No space left on deviceerrno29 :    Illegal seekerrno30 :    Read-only file systemerrno31 :    Too many linkserrno32 :    Broken pipeerrno33 :    Numerical argument out of domainerrno34 :    Numerical result out of rangeerrno35 :    Resource deadlock avoidederrno36 :    File name too longerrno37 :    No locks availableerrno38 :    Function not implementederrno39 :    Directory not emptyerrno40 :    Too many levels of symbolic linkserrno41 :    Unknown error 41errno42 :    No message of desired typeerrno43 :    Identifier removederrno44 :    Channel number out of rangeerrno45 :    Level 2 not synchronizederrno46 :    Level 3 haltederrno47 :    Level 3 reseterrno48 :    Link number out of rangeerrno49 :    Protocol driver not attachederrno50 :    No CSI structure availableerrno51 :    Level 2 haltederrno52 :    Invalid exchangeerrno53 :    Invalid request descriptorerrno54 :    Exchange fullerrno55 :    No anodeerrno56 :    Invalid request codeerrno57 :    Invalid sloterrno58 :    Unknown error 58errno59 :    Bad font file formaterrno60 :    Device not a streamerrno61 :    No data availableerrno62 :    Timer expirederrno63 :    Out of streams resourceserrno64 :    Machine is not on the networkerrno65 :    Package not installederrno66 :    Object is remoteerrno67 :    Link has been severederrno68 :    Advertise errorerrno69 :    Srmount errorerrno70 :    Communication error on senderrno71 :    Protocol errorerrno72 :    Multihop attemptederrno73 :    RFS specific errorerrno74 :    Bad messageerrno75 :    Value too large for defined datatypeerrno76 :    Name not unique on networkerrno77 :    File descriptor in bad stateerrno78 :    Remote address changederrno79 :    Can not access a needed sharedlibraryerrno80 :    Accessing a corrupted sharedlibraryerrno81 :    .lib section in a.out corruptederrno82 :    Attempting to link in too manyshared librarieserrno83 :    Cannot exec a shared librarydirectlyerrno84 :    Invalid or incomplete multibyte orwide charactererrno85 :    Interrupted system call should berestartederrno86 :    Streams pipe errorerrno87 :    Too many userserrno88 :    Socket operation on non-socketerrno89 :    Destinationaddress requirederrno90 :    Message too longerrno91 :    Protocol wrong type for socketerrno92 :    Protocol not availableerrno93 :    Protocol not supportederrno94 :    Socket type not supportederrno95 :    Operation not supportederrno96 :    Protocol family not supportederrno97 :    Address family not supported byprotocolerrno98 :    Address already in useerrno99 :    Cannot assign requested addresserrno100 :   Network is downerrno101 :   Network is unreachableerrno102 :   Network dropped connection onreseterrno103 :   Software caused connection aborterrno104 :   Connection reset by peererrno105 :   No buffer space availableerrno106 :   Transport endpoint is alreadyconnectederrno107 :   Transport endpoint is notconnectederrno108 :   Cannot send after transportendpoint shutdownerrno109 :   Too many references: cannot spliceerrno110 :   Connection timed outerrno111 :   Connection refusederrno112 :   Host is downerrno113 :   No route to hosterrno114 :   Operation already in progresserrno115 :   Operation now in progresserrno116 :   Stale NFS file handleerrno117 :   Structure needs cleaningerrno118 :   Not a XENIX named type fileerrno119 :   No XENIX semaphores availableerrno120 :   Is a named type fileerrno121 :   Remote I/O errorerrno122 :   Disk quota exceedederrno123 :   No medium founderrno124 :   Wrong medium typeerrno125 :   Operation cancelederrno126 :   Required key not availableerrno127 :   Key has expirederrno128 :   Key has been revokederrno129 :   Key was rejected by serviceerrno130 :   Owner diederrno131 :   State not recoverableerrno132 :   Operation not possible due toRF-killerrno133 :   Unknown error 133errno134 :   Unknown error 134errno135 :   Unknown error 135errno136 :   Unknown error 136errno137 :   Unknown error 137errno138 :   Unknown error 138errno139 :   Unknown error 139由上可见Linux对错误宏的定义。头文件 /usr/include/asm-generic/errno-base.h 的源码：#ifndef _ASM_GENERIC_ERRNO_BASE_H
#define _ASM_GENERIC_ERRNO_BASE_H#define EPERM   1 /* Operation not permitted */
#define ENOENT2/* No such file or directory */
#define ESRCH   3 /* No such process */
#define EINTR   4 /* Interrupted system call */
#define EIO   5 /* I/O error */
#define ENXIO   6 /* No such device or address */
#define E2BIG   7 /* Argument list too long */
#define ENOEXEC8/* Exec format error */
#define EBADF   9 /* Bad file number */
#define ECHILD10/* No child processes */
#define EAGAIN11/* Try again */
#define ENOMEM12/* Out of memory */
#define EACCES13/* Permission denied */
#define EFAULT14/* Bad address */
#define ENOTBLK15/* Block device required */
#define EBUSY   16 /* Device or resource busy */
#define EEXIST17/* File exists */
#define EXDEV   18 /* Cross-device link */
#define ENODEV19/* No such device */
#define ENOTDIR20/* Not a directory */
#define EISDIR21/* Is a directory */
#define EINVAL22/* Invalid argument */
#define ENFILE23/* File table overflow */
#define EMFILE24/* Too many open files */
#define ENOTTY25/* Not a typewriter */
#define ETXTBSY26/* Text file busy */
#define EFBIG   27 /* File too large */
#define ENOSPC28/* No space left on device */
#define ESPIPE29/* Illegal seek */
#define EROFS   30 /* Read-only file system */
#define EMLINK31/* Too many links */
#define EPIPE   32 /* Broken pipe */
#define EDOM   33 /* Math argument out of domain of func */
#define ERANGE34/* Math result not representable */#endif头文件/usr/include/asm-generic/erno.h源码：#ifndef _ASM_GENERIC_ERRNO_H
#define _ASM_GENERIC_ERRNO_H#include <asm-generic/errno-base.h>#define EDEADLK35/* Resource deadlock would occur */
#define ENAMETOOLONG36/* File name too long */
#define ENOLCK37/* No record locks available */
#define ENOSYS38/* Function not implemented */
#define ENOTEMPTY39/* Directory not empty */
#define ELOOP   40 /* Too many symbolic links encountered */
#define EWOULDBLOCKEAGAIN/* Operation would block */
#define ENOMSG42/* No message of desired type */
#define EIDRM   43 /* Identifier removed */
#define ECHRNG44/* Channel number out of range */
#define EL2NSYNC45/* Level 2 not synchronized */
#define EL3HLT46/* Level 3 halted */
#define EL3RST47/* Level 3 reset */
#define ELNRNG48/* Link number out of range */
#define EUNATCH49/* Protocol driver not attached */
#define ENOCSI50/* No CSI structure available */
#define EL2HLT51/* Level 2 halted */
#define EBADE   52 /* Invalid exchange */
#define EBADR   53 /* Invalid request descriptor */
#define EXFULL54/* Exchange full */
#define ENOANO55/* No anode */
#define EBADRQC56/* Invalid request code */
#define EBADSLT57/* Invalid slot */#define EDEADLOCKEDEADLK#define EBFONT59/* Bad font file format */
#define ENOSTR60/* Device not a stream */
#define ENODATA61/* No data available */
#define ETIME   62 /* Timer expired */
#define ENOSR   63 /* Out of streams resources */
#define ENONET64/* Machine is not on the network */
#define ENOPKG65/* Package not installed */
#define EREMOTE66/* Object is remote */
#define ENOLINK67/* Link has been severed */
#define EADV   68 /* Advertise error */
#define ESRMNT69/* Srmount error */
#define ECOMM   70 /* Communication error on send */
#define EPROTO71/* Protocol error */
#define EMULTIHOP72/* Multihop attempted */
#define EDOTDOT73/* RFS specific error */
#define EBADMSG74/* Not a data message */
#define EOVERFLOW75/* Value too large for defined data type */
#define ENOTUNIQ76/* Name not unique on network */
#define EBADFD77/* File descriptor in bad state */
#define EREMCHG78/* Remote address changed */
#define ELIBACC79/* Can not access a needed shared library */
#define ELIBBAD80/* Accessing a corrupted shared library */
#define ELIBSCN81/* .lib section in a.out corrupted */
#define ELIBMAX82/* Attempting to link in too many shared libraries */
#define ELIBEXEC83/* Cannot exec a shared library directly */
#define EILSEQ84/* Illegal byte sequence */
#define ERESTART85/* Interrupted system call should be restarted */
#define ESTRPIPE86/* Streams pipe error */
#define EUSERS87/* Too many users */
#define ENOTSOCK88/* Socket operation on non-socket */
#define EDESTADDRREQ89/* Destination address required */
#define EMSGSIZE90/* Message too long */
#define EPROTOTYPE91/* Protocol wrong type for socket */
#define ENOPROTOOPT92/* Protocol not available */
#define EPROTONOSUPPORT93/* Protocol not supported */
#define ESOCKTNOSUPPORT94/* Socket type not supported */
#define EOPNOTSUPP95/* Operation not supported on transport endpoint */
#define EPFNOSUPPORT96/* Protocol family not supported */
#define EAFNOSUPPORT97/* Address family not supported by protocol */
#define EADDRINUSE98/* Address already in use */
#define EADDRNOTAVAIL99/* Cannot assign requested address */
#define ENETDOWN100/* Network is down */
#define ENETUNREACH101/* Network is unreachable */
#define ENETRESET102/* Network dropped connection because of reset */
#define ECONNABORTED103/* Software caused connection abort */
#define ECONNRESET104/* Connection reset by peer */
#define ENOBUFS105/* No buffer space available */
#define EISCONN106/* Transport endpoint is already connected */
#define ENOTCONN107/* Transport endpoint is not connected */
#define ESHUTDOWN108/* Cannot send after transport endpoint shutdown */
#define ETOOMANYREFS109/* Too many references: cannot splice */
#define ETIMEDOUT110/* Connection timed out */
#define ECONNREFUSED111/* Connection refused */
#define EHOSTDOWN112/* Host is down */
#define EHOSTUNREACH113/* No route to host */
#define EALREADY114/* Operation already in progress */
#define EINPROGRESS115/* Operation now in progress */
#define ESTALE116/* Stale NFS file handle */
#define EUCLEAN117/* Structure needs cleaning */
#define ENOTNAM118/* Not a XENIX named type file */
#define ENAVAIL119/* No XENIX semaphores available */
#define EISNAM120/* Is a named type file */
#define EREMOTEIO121/* Remote I/O error */
#define EDQUOT122/* Quota exceeded */#define ENOMEDIUM123/* No medium found */
#define EMEDIUMTYPE124/* Wrong medium type */
#define ECANCELED125/* Operation Canceled */
#define ENOKEY126/* Required key not available */
#define EKEYEXPIRED127/* Key has expired */
#define EKEYREVOKED128/* Key has been revoked */
#define EKEYREJECTED129/* Key was rejected by service *//* for robust mutexes */
#define EOWNERDEAD130/* Owner died */
#define ENOTRECOVERABLE131/* State not recoverable */#define ERFKILL 132/* Operation not possible due to RF-kill */#endif

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